A repeating pulsed field system has been developed for the neutron diffraction experiments in high magnetic fields. Magnetic fields as high as 30 T can be generated by using the present system. Magnetic fields above 25 T have not been obtained by way of a superconducting magnet. Therefore, the present field intensity is the highest field for neutron diffraction in the world. As the magnet, 3-layer Bitter type magnet is used. The use of the 3-layer structure improves the electric and mechanical contacts between the adjacent layers and the loge life-time of magnet is achieved.As the first test of the neutron diffraction experiments above 25 T, we have investigated the metamagnetic phase transition in a triangular lattice anti-ferromagnet CuFeO2. The change of the magnetic structure from the incommensurate to the commensurate structure was found around 20 T. The neutron diffraction experiments in Dy Cu was also made and the nature of metamagentic transition was investigated. The present results have established the feasibility of the neutron diffraction experiments in very high fields above 20 T by using a repeating pulsed magnetic field system.